Transthyretin (TTR) is a carrier protein that is involved in the binding and transport of thyroid hormone and the retinol binding protein/vitamin A complex. TTR is normally present in human plasma and cerobrospinal fluid as a soluble, tetrameric protein, but in several disease conditions can be converted into insoluble amyloid. More than 80 point mutations in TTR have been described and implicated in a group of diseases known collectively as familial amyloid polyneuropathy (FAP). Previous studies have shown that dissociation of the tetramer is rate-limiting for fibril formation in vitro; furthermore, the severity of FAP disease is correlated with the thermodynamic and kinetic stability of the variant tetramers. This study will focus on the mechanism of fibril formation by wild-type and variant TTR using engineered monomeric forms of TTR. The kinetics of the reaction will be examined, and intermediates in the pathway from amyloidogenic monomers to mature fibrils will be characterized using turbidity and dye-binding assays, electron microscopy, analytical ultracentrifugation, and time-lapse atomic force microscopy. These studies will address whether fibrillization is a nucleation dependent polymerization that can be seeded in vivo and whether FAP-variants differ from wild-type TTR in the fibril formation pathway. The results will enhance our understanding of TTR amyloid disease and may shed light on the nature of the intermediates responsible for cellular toxicity. [unreadable] [unreadable]